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« on: August 18, 2018, 01:09:55 AM »
I flew three UAV surveys in the same manner as I always do. For two of them, after aligning images, the estimated camera heights are around 20 m AGL (also ~20 m NAVD88), when they should be around 120 m. The third survey was fine, camera estimates were within 5 m of 120 which is expected.
I tried running it raw (WGS coordinates with awful DJI-recorded height) and had this issue. The issue persisted even when correcting the flight heights: I converted coordinates to UTM. I converted heights to m NAVD88 by first taking offset between photo taken on ground right before launch and first survey photo taken. Offset is added to height of survey-grade GPS point taken at launch point. The issue also persisted when I placed ground control, unchecked camera coordinates, and ran alignment using referenced and also not referenced pre-selection.
The resulting sparse cloud is scaled correctly relative to itself, but the unplaced ground control are below it. Makes it hard to place them at first, but after placing one point on two images it guesses almost perfectly on the remaining images. When I placed ~30 ground control points the estimated camera height rose to 50-70 m AGL, still very low, but the ground control fit much better. The ground control z errors are 2-4 meters, which is unusually bad for this survey style and it's clear that the DSM will be warped by this amount. For the third survey which did not have this height problem, the marker total error is below 1 m which is expected.
I've been using this setup for many surveys and never had an issue until now.
Survey setup:
Phantom 3 Advanced - off the shelf setup
Camera GPS - Phantom 3 Advanced GPS - heights adjusted to NAVD88
Ground control - RTK-GNSS
DJI GS Pro app - flight heights held to 120 m AGL
Sidelap: 70%
Frontlap: 80%
3 flights done in succession over about 1 hour time period
Software setup (Tried alignments using these different settings and got the same error regardless):
Agisoft Photoscan 1.2.4 build 2399
~1000 photos
Alignment: high and highest quality
Referenced and non-referenced resulted in the same error (also tried unchecking camera coordinates, placed ~30 GCPs evenly spread)
Key point limits: tried 8000, 40000, 80000, and 0
Tie point limits: tried 40000, 400000, and 0
Mask: about 10% of image from edges, and no mask
Focal length: 3.61 mm
Pixel size: 0.00156192 mm
Frame camera
Any idea what could be causing this estimated height offset of 100 m, and a way to fix it?